Unsaturated carboxylic acids such as acrylic acid, methacrylic acid and the like are industrially important, as starting materials for various synthetic resins, coatings, plasticizing agents and the like. In particular, the importance of acrylic acid as a starting material for water-absorbent resin has been increasing in late year. As production processes for unsaturated carboxylic acids such as acrylic acid and methacrylic acid, referring to acrylic acid for example, the most commonly practiced is the two-stage oxidation method comprising producing acrolein first by catalytic vapor-phase oxidation of propylene, and then producing acrylic acid by catalytic vapor-phase oxidation of the acrolein. Whereas, development of single-stage oxidation of propane to produce acrylic acid is also advanced in recent years, because propane is cheaper than propylene, and various proposals were made concerning the technology. Also about industrial manufacture of methacrylic acid, known is a two-stage oxidation method comprising first producing methacrolein through catalytic vapor-phase oxidation of at least one starting material selected from isobutylene, t-butanol and methyl-t-butyl ether and then producing methacrylic acid through further catalytic vapor-phase oxidation of the resulting methacrolein.
As the catalysts useful in such catalytic vapor-phase oxidation of unsaturated aldehydes such as acrolein or methacrolein, or saturated hydrocarbons such as propane in the presence of molecular oxygen to make corresponding unsaturated carboxylic acids such as acrylic acid and methacrylic acid, those containing molybdenum and vanadium are widely known. Improvements in not only the yield of the object product but in mechanical strength of the catalysts which is required in the occasions of their industrial use are challenged in the concerned art, and many proposals have been made for the purpose of improving the mechanical strength without impairing yields of the object products.
Such past proposals include, for example, a catalyst obtained by molding and calcining a dry material obtained of a liquid mixture of starting materials, in which the ignition loss ratio of the dry material ranges 5-40 mass % (cf. JP 2004-243213A); an extrusion-molded catalyst containing 0.5-5 wt % of graphite (cf. JP 60 (1985)-150834A); a catalyst containing 0.05-10 wt % of a carbon fiber having an average diameter of 1-20 μm, average length of 10-3,000 μm and a carbon content of at least 93% (cf. JP 7 (1995)-251075A); or a catalyst produced by mixing a precursor of an oxide and the oxide, and calcining the same (JP 2004-351297A).